Photoacoustic image generation apparatus

ABSTRACT

An insertion needle has a light guide member for guiding light emitted from a light source, a light emitting portion for emitting light guided by the light guide member, and a photoacoustic wave generating portion for generating photoacoustic waves caused by light emitted from the light emitting portion. A photoacoustic image generation unit generates a photoacoustic image based on the detection signal of the photoacoustic waves. A distal end candidate extraction unit extracts a distal end candidate region from the shallow side of the image based on the strength of the detection signal of the photoacoustic waves. An image output unit displays the extracted distal end candidate region on an image display unit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of PCT InternationalApplication No. PCT/JP2015/004863 filed on Sep. 24, 2015, which claimspriority under 35 U.S.C. § 119(a) to Japanese Patent Application No.2014-198885 filed on Sep. 29, 2014. Each of the above applications ishereby expressly incorporated by reference, in its entirety, into thepresent application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoacoustic image generationapparatus, more specifically, to a photoacoustic image generationapparatus for generating a photoacoustic image by detectingphotoacoustic waves generated in a subject after emitting light to thesubject.

2. Description of the Related Art

As a kind of image examination method capable of examining the state ofthe inside of a living body in a non-invasive manner, an ultrasoundexamination method is known. In ultrasound examination, an ultrasoundprobe capable of transmitting and receiving ultrasound waves is used.When ultrasound waves are transmitted to the subject (living body) fromthe ultrasound probe, the ultrasound waves propagate through the livingbody to be reflected on the tissue interface. By receiving the reflectedultrasound waves using the ultrasound probe and calculating the distancebased on the time until the reflected ultrasound waves return to theultrasound probe, it is possible to image the state of the inside.

In addition, photoacoustic imaging for imaging the inside of the livingbody using the photoacoustic effect is known. In general, inphotoacoustic imaging, pulsed laser light, such as a laser pulse, isemitted into the body. In the living body, a living tissue absorbs theenergy of the pulsed laser light, and ultrasound waves (photoacousticwaves) due to adiabatic expansion due to the energy are generated. Bydetecting the photoacoustic waves using an ultrasound probe or the likeand forming a photoacoustic image based on the detection signal, it ispossible to visualize the inside of the living body based on thephotoacoustic waves.

For photoacoustic imaging, JP2009-31262A discloses a combination ofphotoacoustic imaging and treatment using an insertion needle. InJP2009-31262A, an affected part such as a tumor, a part suspected to bean affected part, or the like is found by generating a photoacousticimage and observing the image. In order to examine such a part moreprecisely or in order to perform injection into the affected part,sampling of cells, injection into the affected part, and the like areperformed using an insertion needle, such as an injection needle or acytodiagnosis needle. In JP2009-31262A, it is possible to performinsertion while observing the affected part using a photoacoustic image.

In addition, JP2013-13713A also discloses a combination of photoacousticimaging and an insertion needle. In JP2013-13713A, the insertion needlehas a light emitting portion. Light emitted from a laser light source isguided to the light emitting portion of the insertion needle using, forexample, an optical fiber, and is emitted to the outside from the lightemitting portion. By detecting photoacoustic waves, which are generatedby absorbing the light emitted from the light emitting portion of theinsertion needle, using an ultrasound probe and generating aphotoacoustic image based on the detection signal, it is possible tocheck the position of the insertion needle.

Here, the success of the brachial plexus block is largely based on thelocalization of the nerve, the position of a needle, and an appropriatetechnique for local anesthetic injection. In recent years, nerve blockinjection is performed by inserting an insertion needle while observingan ultrasound image. However, there is a problem that it is difficultfor the insertion needle to be visually recognized only with theultrasound image. In the insertion, it is important that the entireneedle can be seen. However, in order to prevent pneumothorax and thelike, it is most important to check the position of the distal end ofthe needle. In photoacoustic imaging, usually, emission of light to thesubject is performed from the surface of the subject. In particular,when the distal end of the insertion needle is inserted up to a deepposition (for example, a position deeper than 3 cm from the subjectsurface), light emitted from the subject surface does not sufficientlyreach the insertion needle that has been inserted to the deep position.Accordingly, it is difficult to check the position of the distal end ofthe insertion needle in a photoacoustic image.

To solve the problem, there is a technique disclosed in WO2014/109148A.In WO2014/109148A, light emitted from the light source is guided to thevicinity of the distal end of the insertion needle using an opticalfiber or the like, and the light is emitted to a photoacoustic wavegenerating portion of the insertion needle from there. In this manner,it is possible to check the position using a photoacoustic image evenwhen the insertion needle is inserted up to a deep position.

SUMMARY OF THE INVENTION

In WO2014/109148A, since the photoacoustic wave generating portion isprovided at the distal end of the insertion needle, it is possible togenerate photoacoustic waves at one point of the distal end of theinsertion needle. However, the photoacoustic waves generated at thedistal end of the insertion needle may be multi-reflected by the mainbody of the insertion needle and the bone, tissues, and the like, whichare present around the main body of the insertion needle and have highreflectances of acoustic waves. Such multi-reflections cause artifactsin a photoacoustic image (sound artifacts). In addition, when the lightemitted to the photoacoustic wave generating portion is emitted to bloodvessels or the like present in the insertion direction of the insertionneedle, photoacoustic waves are generated in the blood vessels or thelike, and such photoacoustic waves also cause artifacts (lightartifacts), These artifacts are obstacles to checking the position ofthe insertion needle using a photoacoustic image. The above problemsalso occur similarly in the case of checking the position of an insertother than the insertion needle.

In view of the above, it is an object of the present invention toprovide a photoacoustic image generation apparatus capable of easilychecking the position of an insert by suppressing the influence ofartifacts.

In order to achieve the aforementioned object, the present inventionprovides a photoacoustic image generation apparatus comprising: aninsert at least a part of which is inserted into a subject and which hasa light guide member for guiding light emitted from a light source, alight emitting portion for emitting light guided by the light guidemember, and a photoacoustic wave generating portion for generatingphotoacoustic waves caused by light emitted from the light emittingportion; acoustic wave detection means for detecting photoacoustic wavesemitted from the insert; photoacoustic image generation means forgenerating a photoacoustic image based on a detection signal of thephotoacoustic waves; distal end candidate extraction means forextracting a distal end candidate region from a shallow side based on astrength of the detection signal of the photoacoustic waves; and imageoutput means for displaying the extracted distal end candidate region onimage display means.

In the photoacoustic image generation apparatus of the presentinvention, it is preferable that the distal end candidate extractionmeans extracts a distal end candidate region with a predetermined numberas an upper limit.

It is preferable that the photoacoustic image generation apparatus ofthe present invention further has correction means for suppressing astrength of a detection signal of photoacoustic waves in a distal endcandidate region other than a distal end candidate region located in ashallowest portion, in a photoacoustic image, relative to a strength ofa detection signal of photoacoustic waves in the distal end candidateregion located in the shallowest portion.

With the distal end candidate region located in the shallowest portionamong the extracted distal end candidate regions as a reference, thecorrection means may correct a strength of a detection signal ofphotoacoustic waves in each distal end candidate region using acoefficient according to a distance in a depth direction between eachdistal end candidate region and the reference distal end candidateregion.

It is preferable that the correction means emphasizes the distal endcandidate region located in the shallowest portion relative to remainingdistal end candidate regions.

The distance may be defined as a distance in the depth direction betweena center-of-gravity position of the reference distal end candidateregion and each of remaining distal end candidate regions.

The coefficient is a maximum when the distance is 0, and a value of thecoefficient decreases as the distance increases.

The distal end candidate extraction means may extract, as a distal endcandidate region, a region where the strength of the detection signal ofthe photoacoustic waves is equal to or greater than a threshold value.

The distal end candidate extraction means may extract a distal endcandidate region based not only on the strength of the detection signalof the photoacoustic waves but also on an area of a region where thestrength of the detection signal of the photoacoustic waves is equal toor greater than a threshold value.

The image output means may mask a region other than the extracted distalend candidate region in the photoacoustic image.

It is preferable that the distal end candidate extraction means extractsthe distal end candidate region after performing smoothing processing onthe detection signal of the photoacoustic waves.

The insert may have an opening, and have an inner cavity thereinside.

The photoacoustic wave generating portion may include a light absorptionmember that absorbs the light emitted from the light emitting portion togenerate photoacoustic waves.

The insert may be a needle inserted into a subject.

The light emitting portion may emit at least some of light beams guidedby the light guide member toward an inner wall of the inner cavity.

The acoustic wave detection means may further detect reflected acousticwaves of acoustic waves transmitted toward a subject. In this case, thephotoacoustic image generation apparatus of the present invention mayfurther have reflected acoustic wave image generation means forgenerating a reflected acoustic wave image based on the reflectedacoustic waves.

The image output means may display the extracted distal end candidateregion on the image display means so as to be superimposed on thereflected acoustic wave image.

In the photoacoustic image generation apparatus of the presentinvention, it is easy to check the position of the insert by suppressingthe influence of artifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a photoacoustic image generationapparatus according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing an insertion needle.

FIG. 3 is a flowchart showing an operation procedure.

FIG. 4 is a flowchart showing the procedure of distal end candidateextraction processing.

FIG. 5A is a diagram showing a photoacoustic signal after detection andlogarithmic conversion as an image.

FIG. 5B is a diagram showing an image subjected to smoothing processing.

FIG. 5C is a diagram showing a binary image.

FIG. 5D is a diagram showing an image of an extracted distal endcandidate region.

FIG. 6 is a block diagram showing a photoacoustic image generationapparatus according to a second embodiment of the present invention.

FIG. 7 is a diagram schematically showing a distal end candidate region.

FIG. 8 is a graph showing a specific example of a coefficient.

FIG. 9 is a flowchart showing the procedure of correction of a pixelvalue according to the depth.

FIG. 10A is a diagram showing a photoacoustic image after correction.

FIG. 10B is a diagram showing a photoacoustic image after correction.

FIG. 11 is a cross-sectional view showing the vicinity of the distal endof an insertion needle in a modification example.

FIG. 12 is a diagram showing the appearance of a photoacoustic imagegeneration apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the diagrams. FIG. 1 shows a photoacousticimage generation apparatus according to a first embodiment of thepresent invention. A photoacoustic image generation apparatus 10includes a probe (ultrasound probe) 11, an ultrasound unit 12, a laserunit 13, and an insertion needle 15. In the embodiment of the presentinvention, an ultrasound wave is used as an acoustic wave. However, thepresent invention is not limited to the ultrasound wave, and an acousticwave having an audible frequency may be used as long as an appropriatefrequency can be selected according to an examination target,measurement conditions, or the like.

The laser unit 13 is a light source. Light emitted from the laser unit13 is guided to the insertion needle 15 that is an insert, for example,using light guide means, such as an optical fiber 16. The laser unit 13is a laser diode light source (semiconductor laser light source), forexample. Alternatively, the laser unit 13 may be a light amplificationtype laser light source having a laser diode light source as a seedlight source. Types of light sources are not particularly limited, andthe laser unit 13 may be a solid state laser light source using yttriumaluminum garnet (YAG), alexandrite, and the like. Light sources otherthan the laser light source may be used.

The insertion needle 15 is a needle inserted into the subject. FIG. 2shows a cross section of the insertion needle 15. The insertion needle15 includes: a hollow insertion needle body 151 that has an opening atthe distal end formed at an acute angle and has an inner cavitythereinside; a light guide member 155 for guiding light emitted from thelaser unit 13 to the vicinity of the opening of the insertion needle;and a light absorption member 157 that absorbs laser light emitted fromthe light guide member 155 to generate photoacoustic waves.

The light guide member 155 and the light absorption member 157 aredisposed inside the insertion needle body 151. The light guide member155 is connected to the optical fiber 16 (refer to FIG. 1) through anoptical connector provided in a proximal end portion of the insertionneedle 15, for example. The light guide member 155 is formed of, forexample, an optical fiber, and the end surface of the optical fiber onthe light traveling side when viewed from the laser unit 13 forms alight emitting portion. For example, laser light of 0.2 mJ is emittedfrom the light emitting portion. Instead of providing the opticalconnector, the optical fiber 16 may be inserted into the tube 158 andthe optical fiber 16 itself may be used as the light guide member 155.

The light absorption member 157 is provided at a position to which thelight emitted from the light emitting portion of the light guide member155 is emitted. The light absorption member 157 is provided in thevicinity of the distal end of the insertion needle 15 and on the innerwall of the insertion needle body 151. The light absorption member 157is a photoacoustic wave generating portion that absorbs the lightemitted from the light emitting portion to generate photoacoustic waves.The light absorption member 157 is formed of, for example, an epoxyresin containing black pigment mixed thereinto, a polyurethane resin, afluorine resin or silicone rubber, and black paint having high lightabsorptivity with respect to the wavelength of laser light. In FIG. 2,the light absorption member 157 is drawn larger than the light guidemember 155. However, without being limited to this, the light absorptionmember 157 may have approximately the same size as the diameter of thelight guide member 155.

The light absorption member 157 is not limited to that described above,and a metal film or an oxide film having light absorptivity with respectto the wavelength of laser light may be used as the light absorptionmember 157. For example, a film of an oxide, such as an iron oxide, achromium oxide, and a manganese oxide having high light absorptivitywith respect to the wavelength of laser light, can be used as the lightabsorption member 157. Alternatively, a metal film such as Ti or Ptwhich has lower light absorptivity than oxides and has highbiocompatibility may be used as the light absorption member 157. Theposition where the light absorption member 157 is provided is notlimited to the inner wall of the insertion needle body 151. For example,a metal film or an oxide film that is the light absorption member 157may be formed on the light emitting surface of the light guide member155 in a thickness of, for example, about 100 nm by vapor deposition orthe like, and the metal film or the oxide film may cover the lightemitting surface. In this case, at least some of light beams emittedfrom the light emitting surface of the light guide member 155 areabsorbed by the metal film or the oxide film that covers the lightemitting surface, and photoacoustic waves are generated from the metalfilm or the oxide film.

The “vicinity” of the distal end of the insertion needle 15 means aposition where it is possible to generate photoacoustic waves capable ofimaging the position of the distal end of the insertion needle 15 withaccuracy, which is required for insertion work, in a case where thelight emitting surface of the light guide member 155 and the lightabsorption member 157 are disposed at the position. For example,“vicinity” indicates the range of 0 mm to 3 mm toward the proximal endside from the distal end of the insertion needle 15. Also in subsequentembodiments, the meaning of the vicinity of the distal end is the same.

Referring back to FIG. 1, a probe 11 is acoustic wave detection means,and has a plurality of detector elements (ultrasound transducers)arranged in a one-dimensional manner. The probe 11 detects photoacousticwaves generated from the light absorption member 157 (refer to FIG. 2)after the insertion needle 15 is inserted into the subject. In additionto the detection of photoacoustic waves, the probe 11 performstransmission of acoustic waves (ultrasound waves) to the subject andreception of reflected acoustic waves (reflected ultrasound waves) ofthe transmitted ultrasound waves. In addition, transmission andreception of ultrasound waves may be performed at separate positions.For example, ultrasound waves may be transmitted from a positiondifferent from the probe 11, and reflected ultrasound waves of thetransmitted ultrasound waves may be received by the probe 11. The probe11 is not limited to the linear probe, but may be a convex probe or asector probe.

The ultrasound unit 12 has a receiving circuit 21, a receiving memory22, data separation means 23, photoacoustic image generation means 24,ultrasound image generation means 25, image output means 26, atransmission control circuit 27, control means 28, and distal endcandidate extraction means 29. The ultrasound unit 12 forms a signalprocessing device.

The receiving circuit 21 receives a detection signal output from theprobe 11, and stores the received detection signal in the receivingmemory 22. Typically, the receiving circuit 21 includes a low noiseamplifier, a variable gain amplifier, a low pass filter, and an analogto digital convertor (AD converter). The detection signal of the probe11 is amplified by the low noise amplifier, and then the gain isadjusted according to the depth by the variable gain amplifier and ahigh-frequency component is cut by the low pass filter. Then, conversioninto a digital signal is performed by the AD converter, and the digitalsignal is stored in the receiving memory 22. The receiving circuit 21 isformed by one integral circuit (IC), for example.

The probe 11 outputs a detection signal (also referred to as aphotoacoustic signal) of photoacoustic waves and a detection signal(also referred to as a reflected ultrasound signal) of reflectedultrasound waves, and a photoacoustic signal and a reflected ultrasoundsignal (sampling data thereof) after AD conversion are stored in thereceiving memory 22. The data separation means 23 reads the samplingdata of the photoacoustic signal from the receiving memory 22, andtransmits the sampling data to the photoacoustic image generation means24. In addition, the data separation means 23 reads the sampling data ofthe reflected ultrasound signal from the receiving memory 22, andtransmits the sampling data to the ultrasound image generation means(reflected acoustic wave image generation means) 25.

The photoacoustic image generation means 24 generates a photoacousticimage based on the photoacoustic signal detected by the probe 11. Thegeneration of a photoacoustic image includes, for example, imagereconstruction such as phase matching addition, detection, andlogarithmic conversion. The ultrasound image generation means 25generates an ultrasound image (reflected acoustic wave image) based onthe reflected ultrasound signal detected by the probe 11. The generationof an ultrasound image also includes image reconstruction such as phasematching addition, detection, and logarithmic conversion.

The distal end candidate extraction means 29 extracts a distal endcandidate region from the shallow side of the image based on thestrength of the detection signal of photoacoustic waves. The strength ofthe detection signal of photoacoustic waves corresponds to a pixel value(gradation value) in a photoacoustic image. Therefore, the followingexplanation will be given on the assumption that the distal endcandidate extraction means 29 extracts a distal end candidate regionbased on the pixel value of the photoacoustic image. The distal endcandidate extraction means 29 extracts a distal end candidate regionwith a predetermined number as an upper limit, for example. In aphotoacoustic image, the distal end of the insertion needle 15 ispresent in a portion with a high pixel value (portion where detectedphotoacoustic waves are strong). It is preferable that the distal endcandidate extraction means 29 extracts, as a distal end candidateregion, a region where the pixel value of the photoacoustic image isequal to or greater than a threshold value.

Here, a photoacoustic signal after reconstruction in photoacoustic imagegeneration can be regarded as a photoacoustic image. The distal endcandidate extraction means 29 may extract a distal end candidate regionfrom any image (signal) in the image generation step. Specifically, adistal end candidate region may be extracted from the photoacousticsignal after reconstruction, or a distal end candidate region may beextracted from the photoacoustic signal after detection and logarithmicconversion. Alternatively, a distal end candidate region may beextracted from an image obtained by converting the photoacoustic signalafter detection and logarithmic conversion into display gradation usinga lookup table or the like. From the point of view of ease ofprocessing, it is preferable that the photoacoustic signal afterdetection and logarithmic conversion is subject to the distal endcandidate extraction processing.

The image output means 26 outputs the extracted distal end candidateregion to image display means 14, such as a display device. At thistime, it is preferable that the image output means 26 outputs theextracted distal end candidate region to the image display means 14 in astate in which the distal end candidate region is superimposed on theultrasound image. The image output means 26 may output an image of aportion of the distal end candidate region extracted in thephotoacoustic images to the image display means 14. In other words, theimage output means 26 may mask a region other than the distal endcandidate region extracted in the photoacoustic image.

The control means 28 controls each unit in the ultrasound unit 12. Forexample, in the case of acquiring a photoacoustic image, the controlmeans 28 transmits a trigger signal to the laser unit 13 so that thelaser unit 13 emits laser light. In addition, the control means 28controls the sampling start timing of photoacoustic waves bytransmitting a sampling trigger signal to the receiving circuit 21 inresponse to the emission of the laser light. The area wherephotoacoustic waves are to be detected may be divided into a pluralityof areas. In this case, emission of light to the subject and detectionof photoacoustic waves are performed for each area.

In the case of acquiring an ultrasound image, the control means 28transmits an ultrasound wave transmission trigger signal for giving aninstruction to transmit ultrasound waves to the transmission controlcircuit 27. When the ultrasound wave transmission trigger signal isreceived, the transmission control circuit 27 makes the probe 11transmit ultrasound waves. The probe 11 detects reflected ultrasoundwaves by performing a scan while shifting the acoustic line by one lineat a time, for example. The control means 28 transmits a samplingtrigger signal to the receiving circuit 21 according to the timing ofultrasound wave transmission, thereby starting the sampling of reflectedultrasound waves.

FIG. 3 shows an operation procedure. A doctor or the like inserts theinsertion needle 15 into the subject (step A1). After the insertion ofthe insertion needle 15, the control means 28 transmits an ultrasoundtrigger signal to the transmission control circuit 27. The transmissioncontrol circuit 27 makes the probe 11 transmit ultrasound waves inresponse to the ultrasound trigger signal (step A2). The probe 11detects reflected ultrasound waves after the transmission of ultrasoundwaves (step A3). In addition, transmission and reception of ultrasoundwaves may be performed at separate positions. For example, ultrasoundwaves may be transmitted from a position different from the probe 11,and reflected ultrasound waves of the transmitted ultrasound waves maybe received by the probe 11.

The reflected ultrasound signal output from the probe 11 is stored inthe receiving memory 22 through the receiving circuit 21. The dataseparation means 23 transmits the reflected ultrasound signal stored inthe receiving memory 22 to the ultrasound image generation means 25. Theultrasound image generation means 25 generates an ultrasound image basedon the reflected ultrasound signal (step A4).

The control means 28 of the ultrasound unit 12 transmits a triggersignal to the laser unit 13. When the trigger signal is received, thelaser unit 13 starts laser oscillation to emit pulsed laser light (stepA5). The pulsed laser light emitted from the laser unit 13 is guided tothe vicinity of the distal end of the insertion needle 15 by the lightguide member 155 (refer to FIG. 2), and is emitted from the lightabsorption member 157.

The probe 11 detects photoacoustic waves generated in the subject due tothe emission of the laser light (step A6). The receiving circuit 21receives the photoacoustic signal from the probe 11, and stores thesampling data of the photoacoustic signal in the receiving memory 22.Here, photoacoustic waves propagate one way from the vicinity of thedistal end of the insertion needle 15, which is a generation position ofthe photoacoustic waves, to the probe 11, while the reflected ultrasoundwaves transmitted from the probe 11 propagate back and forth between theprobe 11 and the ultrasound wave reflection position. Accordingly, thedetection of the reflected ultrasound waves requires twice the time forthe detection of the photoacoustic waves generated at the same depthposition. For this reason, the sampling clock of the AD converter at thetime of reflected ultrasound wave sampling may be half at the time ofphotoacoustic wave sampling.

The data separation means 23 transmits the photoacoustic signal storedin the receiving memory 22 to the photoacoustic image generation means24. The photoacoustic image generation means 24 generates aphotoacoustic image based on the photoacoustic signal (step A7). Thedistal end candidate extraction means 29 extracts a region, which is adistal end candidate of the insertion needle 15, from the photoacousticimage (step A8). The image output means 26 displays, on the imagedisplay means 14, an image obtained by superimposing an image, which isobtained by masking a region other than the distal end candidate regionextracted in step A8 in the photoacoustic image generated in step A7, onthe ultrasound image generated in step A4 (step A9).

FIG. 4 shows the procedure of the distal end candidate extractionprocess. Here, explanation will be given on the assumption that a distalend candidate region is extracted based on the photoacoustic signalafter detection and logarithmic conversion. The distal end candidateextraction means 29 performs smoothing processing on the photoacousticsignal (photoacoustic image) after detection and logarithmic conversion(step B1). As the smoothing processing, for example, filter processingusing a Gaussian filter can be used. It is preferable that the filtersize of the Gaussian filter is smaller than the size of the distal endportion of the insertion needle 15.

The distal end candidate extraction means 29 binarizes a photoacousticsignal after the smoothing processing (step B2). By the binarization,the photoacoustic signal is divided into a portion where the signalstrength is equal to or greater than a threshold value and a portionwhere the signal strength is less than the threshold value. After thebinarization, a region where pixels are continuous is extracted as adistal end candidate region (step B3). At this time, the distal endcandidate extraction means 29 may extract a region where pixels arecontinuous by a predetermined number from the shallow side. When adistal end candidate region is extracted from the shallow side and thenumber of extracted distal end candidate regions reaches a predeterminednumber, the extraction of distal end candidate regions are ended. Whenthe number of regions where pixels are continuous is less than thepredetermined number, the region extraction may be ended at that pointin time.

FIG. 5A shows a photoacoustic signal after detection and logarithmicconversion as an image, and FIG. 5B is an image obtained by performingsmoothing processing on the image. FIG. 5C is a binary image, and FIG.5D is an image of an extracted distal end candidate region. In theseimages, an upper portion of the paper corresponds to the shallow side ofthe subject, and the depth increases downward.

In FIG. 5A, a portion where the detected photoacoustic signal is thestrongest, that is, a portion where the pixel value of the photoacousticimage is the largest corresponds to a position where the lightabsorption member 157 (refer to FIG. 2) is present in the insertionneedle 15. Although it is ideal that photoacoustic waves are detectedonly at the position where the light absorption member 157 is present,the actual detection signal includes a noise component or a false signalas shown in FIG. 5A. The false signal causes artifacts. Referring toFIG. 5A, in particular, a false signal is present in a portion deeperthan the distal end portion of the insertion needle 15 of thephotoacoustic image.

Referring to FIG. 5B, a noise component is remove by performingsmoothing processing on the photoacoustic image shown in FIG. 5A. When abinary image, in which a pixel whose signal strength of thephotoacoustic signal is equal to or greater than a threshold value isexpressed in white and a pixel whose signal strength of thephotoacoustic signal is smaller than the threshold value is expressed inblack, is generated by performing binarization processing on thephotoacoustic image after the smoothing processing shown in FIG. 5B, theimage shown in FIG. 5C is obtained. From such a binary image, a regionwhere white pixels are continuous is extracted as a distal end candidateregion from the shallow side of the image. In FIG. 5C, regions 201, 202,and 203 are extracted as distal end candidates.

When a region other than the extracted distal end candidate regions inthe photoacoustic image after the smoothing processing shown in FIG. 5Bis masked, the image shown in FIG. 5D is obtained. For example, theimage shown in FIG. 5D is displayed on the image display means 14. Bydisplaying only a portion corresponding to the distal end candidateregion in the photoacoustic image on the image display means 14, extrainformation is deleted from the photoacoustic image. Accordingly, thedoctor or the like can check the position of the insertion needle 15without being confused by artifacts.

Here, in a case where the light absorption member 157 (refer to FIG. 2)is provided in the insertion needle 15 and light is emitted to the lightabsorption member 157 to generate photoacoustic waves, the photoacousticwave generation source is only at the position where the lightabsorption member 157 is present. In practice, however, since lightartifacts or sound artifacts are caused, a photoacoustic image isgenerated as if photoacoustic waves have been detected from a pluralityof positions.

The present inventors have ascertained that all artifacts appear atdeeper positions than the actual photoacoustic wave generation source,and have thought that it is necessary to extract a distal end candidateregion from the shallow side of the image in order to suppressartifacts. In the present embodiment, the distal end candidateextraction means 29 extracts a distal end candidate region from theshallow side of the photoacoustic image based on the pixel value of thephotoacoustic image. By displaying the extracted distal end candidateregion, artifacts are suppressed. Therefore, it becomes easy to checkthe position of the insertion needle 15.

In particular, by setting the upper limit of the number of distal endcandidate regions to be extracted and extracting the distal endcandidate regions from the shallow side with a predetermined number asan upper limit, artifacts at deep positions are hardly extracted asdistal end candidate regions. By suppressing the display of artifacts atdeep positions, it becomes easy to check the position of the insertionneedle 15.

Next, a second embodiment of the present invention will be described.FIG. 6 shows the photoacoustic image generation apparatus according tothe second embodiment of the present invention. A photoacoustic imagegeneration apparatus 10 a according to the present embodiment isdifferent from the photoacoustic image generation apparatus 10 accordingto the first embodiment shown in FIG. 1 in that a correction means 30 isadded to an ultrasound unit 12 a. Others may be the same as in the firstembodiment.

The correction means 30 receives information regarding the extracteddistal end candidate region from the distal end candidate extractionmeans 29. The correction means 30 corrects the photoacoustic image basedon the received information. More specifically, a distal end candidateregion other than a distal end candidate region located in theshallowest portion in the photoacoustic image is suppressed relative tothe distal end candidate region located in the shallowest portion. Thecorrection means 30 may correct a photoacoustic image for displaygenerated by the photoacoustic image generation means 24, or may correcta photoacoustic signal in the photoacoustic image generation step,specifically, a photoacoustic signal after reconstruction or aphotoacoustic signal after detection and logarithmic conversion.

For example, for a distal end candidate region excluding a distal endcandidate region located in the shallowest portion among the extracteddistal end candidate regions, the correction means 30 corrects the pixelvalue in each distal end candidate region using a coefficientcorresponding to the depth direction between the distal end candidateregion excluding the distal end candidate region located in theshallowest portion and the distal end candidate region located in theshallowest portion. The coefficient used for correction is the maximumwhen the distance is 0, and the value decreases as the distanceincreases. By the multiplication of such a coefficient, the distal endcandidate region located in the shallowest portion in the photoacousticimage can be emphasized relative to the remaining distal end candidateregions.

FIG. 7 schematically shows the distal end candidate region shown in FIG.5C. In FIG. 5C, among the extracted three distal end candidate regions,the distal end candidate region 201 is present at the shallowestposition. The correction means 30 calculates a distance in the depthdirection between the remaining distal end candidate regions 202 and 203with the distal end candidate region 201 as a reference. Referring toFIG. 7, the distance in the depth direction between the distal endcandidate regions 201 and 202 is d1, and the distance in the depthdirection between the distal end candidate regions 201 and 203 is d2.The distance between distal end candidate regions can be defined as adistance between the center-of-gravity positions of respective regions.

FIG. 8 shows a specific example of a coefficient. The horizontal axis ofthe graph shown in FIG. 8 is a distance in the depth direction, and thevertical axis is a coefficient. The coefficient is a function of adistance d, and is expressed as f(d). The coefficient f(d) is a functionthat monotonically decreases with the distance d. f(d) is the maximumwhen the distance d is 0, and the value decreases as the distance dincreases from the point where the distance d has increased to someextent. For example, when the distance d is 1 cm, the value of f(d) maybe about half of the maximum value. f(d) may decrease linearly withrespect to the distance d, or may decrease quadratically. Alternatively,f(d) may decrease stepwise.

FIG. 9 shows a procedure for correcting the pixel value according to thedepth. The process shown in FIG. 9 is performed after step A8 in FIG. 3.The correction means 30 calculates a distance in the depth directionbetween a distal end candidate region located in the shallowest portion,among the distal end candidate regions extracted by the distal endcandidate extraction means 29, and each of the remaining distal endcandidate regions (step C1). In step C1, for example, a distance d1between the distal end candidate regions 201 and 202 shown in FIG. 7 anda distance d2 between the distal end candidate regions 201 and 203 arecalculated. The correction means 30 suppresses the pixel value of thedistal end candidate region according to the depth calculated in step C1(step C2).

For example, it is assumed that the correction means 30 corrects aphotoacoustic signal after detection and logarithmic conversion. Aphotoacoustic signal of a pixel belonging to the distal end candidateregion 201 is set to I_(N), a photoacoustic signal of a pixel belongingto the distal end candidate region 202 is set to I_(A1), and aphotoacoustic signal of a pixel belonging to the distal end candidateregion 203 is set to I_(A2). In this case, for the distal end candidateregion 201, the correction means 30 leaves the photoacoustic signalI_(N) as it is without correcting the photoacoustic signal I_(N). Forthe distal end candidate region 202, the correction means 30 multipliesthe photoacoustic signal I_(A1) corresponding to the depth by thecoefficient f(d1), thereby correcting the photoacoustic signal I_(A1) tof(d1)×I_(A1). For the distal end candidate region 203, the correctionmeans 30 multiplies the photoacoustic signal I_(A2) corresponding to thedepth by the coefficient f(d2), thereby correcting the photoacousticsignal I_(A2) to f(d2)×I_(A2). The photoacoustic signal after correctionis returned to the photoacoustic image generation means 24, and isconverted into display gradation using a lookup table or the like.

FIGS. 10A and 10B show photoacoustic images after correction. FIG. 10Ashows the photoacoustic signal corrected by the correction means 30 asan image, and FIG. 10B shows a photoacoustic image obtained byconverting the corrected photoacoustic signal into display gradation.Since the coefficient f(d) is a monotonically decreasing function, thedistal end candidate region at a deeper position is suppressed.Accordingly, the distal end candidate region located in the shallowestportion is relatively emphasized. By suppressing the pixel value (signalstrength) according to the depth by the correction means 30, artifactsare suppressed as shown in FIGS. 10A and 10B, compared with the imagebefore correction (refer to FIG. 5D). In particular, in FIG. 10B, in theimage converted into display gradation using a lookup table or the like,artifacts are almost invisible.

In the present embodiment, the pixel value of each distal end candidateregion other than the distal end candidate region located in theshallowest portion, among the extracted distal end candidate regions, issuppressed according to the depth by the correction means 30. In a casewhere the light absorption member 157 (refer to FIG. 2) is provided inthe insertion needle 15 and light is emitted to the light absorptionmember 157 to generate photoacoustic waves, artifacts appear at aposition deeper than the light absorption member 157. In thephotoacoustic image, the distal end candidate region located in theshallowest portion is thought to correspond to a region wherephotoacoustic waves are generated in the light absorption member 157,and a distal end candidate region located in a portion deeper than thatis thought to be an artifact. Specifically, it becomes easy to check theposition of the insertion needle 15 by relatively emphasizing the distalend candidate region located in the shallowest portion. Even if there isa portion where the photoacoustic signal is strong in a region shallowerthan a region where photoacoustic waves are generated in the lightabsorption member 157, it is thought that the distance in the depthdirection between the portion and the region where photoacoustic wavesare generated in the light absorption member 157 is not large.Accordingly, it is thought that the region where photoacoustic waves aregenerated in the light absorption member 157 will not be invisible inthe photoacoustic image.

In addition, in each of the embodiments described above, the distal endcandidate extraction means 29 extracts a distal end candidate regionbased on the pixel value of the photoacoustic image (signal strength ofthe photoacoustic signal). However, the distal end candidate extractionmeans 29 may extract a distal end candidate region based not only on thepixel value but also on the area of a region having a pixel value equalto or greater than the threshold value. For example, a function having apixel value and the area of a region, in which the pixel value is equalto or greater than the threshold value, as variables may be prepared asan evaluation function indicating the likelihood of the distal end ofthe insertion needle, and a distal end candidate region may be extractedusing the evaluation function. The evaluation function indicates alarger value as the pixel value of a region becomes higher and the areaof the region becomes larger, and indicates a smaller value as the pixelvalue of a region becomes lower and the area of the region becomessmaller, for example. In this case, the distal end candidate extractionmeans 29 may extract a region having an evaluation value equal to orgreater than the threshold value as a distal end candidate region.

The insertion needle is not limited to that shown in FIG. 2 as long asphotoacoustic waves can be generated by light guided to the insertionneedle. FIG. 11 shows a cross section of the vicinity of the distal endof an insertion needle in a modification example. An insertion needle 15a in this modification example has an insertion needle body 151 formingthe outer needle and an inner needle 152 inserted into the insertionneedle body 151. The inner needle 152 includes a light guide member 155,a light absorption member 157, a tube 158, and a transparent resin 159.The tube 158 is a hollow tube formed of polyimide, for example. The tube158 may be a metal tube formed of stainless steel. The outer diameter ofthe tube 158 is slightly smaller than the diameter of the inner cavityof the insertion needle body 151. The transparent resin 159 is disposedwithin the tube 158. For example, an epoxy resin (adhesive) is used asthe transparent resin 159. The tube 158 and the transparent resin 159are cut at an acute angle similar to the insertion needle tip formed atan acute angle. The transparent resin 159 may clog at least a distal endportion of the tube 158, and does not necessarily need to clog theentire inside of the tube 158. As the transparent resin 159, aphotocurable resin, a thermally curable resin, or a room temperaturecurable resin can be used.

Light guided by the optical fiber 16 (refer to FIG. 1) is incident onthe light guide member 155 in the inner needle 152 from the opticalconnector provided in the proximal end portion of the inner needle, forexample. Instead of providing the optical connector in the proximal endportion of the inner needle, the optical fiber 16 may be inserted intothe tube 158 and the optical fiber 16 itself may be used as the lightguide member 155. The light guide member 155 guides the light emittedfrom the laser unit 13 in the vicinity of the opening of the insertionneedle. The light guided by the light guide member 155 is emitted from alight emitting portion 156 provided in the vicinity of the opening. Thelight guide member 155 is formed of, for example, an optical fiber, andthe end surface of the optical fiber on the light traveling side whenviewed from the laser unit 13 forms the light emitting portion 156. Forexample, laser light of 0.2 mJ is emitted from the light emittingportion 156.

The light guide member 155 is embedded into the tube 158 by thetransparent resin 159. The light absorption member 157 that is aphotoacoustic wave generating portion is disposed at the distal end ofthe tube 158, and the light emitted from the light emitting portion 156is emitted to the light absorption member 157. Due to the absorption ofthe emitted light by the light absorption member 157, photoacousticwaves are generated at the distal end of the insertion needle. Since thelight absorption member 157 is present at the distal end of theinsertion needle 15 a, it is possible to generate photoacoustic waves atone point of the distal end of the insertion needle 15 a. Since thelength of a photoacoustic wave generation source (sound source) issufficiently shorter than the length of the entire insertion needle, thesound source can be regarded as a point source. As the light absorptionmember 157, for example, an epoxy resin containing black pigment mixedthereinto, a polyurethane resin, a fluorine resin, or silicone rubbercan be used. Alternatively, a metal or oxide having a light absorptionproperty with respect to the wavelength of laser light may be used asthe light absorption member 157. For example, oxides, such as an ironoxide, a chromium oxide, and a manganese oxide having a high lightabsorption property with respect to the wavelength of laser light, canbe used as the light absorption member 157. Alternatively, a metal, suchas Ti or Pt, may be used as the light absorption member 157.

The inner needle 152 can be manufactured in the following procedure.First, the transparent resin 159 before curing is injected into the tube158. Then, the light guide member 155 is inserted into the tube 158, andis positioned such that the light emitting end of the light guide member155 forming the light emitting portion 156 is disposed in the vicinityof the tube 158. In this positioning, the position may be adjusted suchthat the light emitting end is disposed at the distal end of the tube158 by observing the light guide member 155 using a microscope, forexample. Here, “vicinity” refers to a position where it is possible togenerate photoacoustic waves capable of imaging the position of thedistal end of the insertion needle with accuracy, which is required forinsertion work in the light absorption member 157 disposed at the distalend, in a case where the light emitting portion 156 is disposed at theposition. For example, “vicinity” is the range of 0 mm to 3 mm towardthe proximal end side from the distal end of the insertion needle. Sincethe transparent resin 159 is transparent, it is possible to check theposition of the light emitting end of the light guide member 155 duringadjustment. Instead of the above, the light guide member 155 may beinserted first, and the transparent resin 159 may be injectedthereafter.

After positioning, the transparent resin 159 is cured by heat curing ina state in which the light guide member 155 has been inserted into thetube 158. Then, the distal ends of the tube 158 and the transparentresin 159 are cut at an acute angle so as to have a shape suitable forthe distal end of the insertion needle body 151. Then, the resin havinga light absorption property that forms the light absorption member 157is applied to cover at least a part of the cut surface, and the resin iscured by heat curing, for example.

In the above, the light guide member 155 is inserted into the tube 158and the position is adjusted, and the transparent resin is cured and isthen cut at an acute angle. However, the invention is not limitedthereto. The tube may be cut at an acute angle first, the light guidemember 155 may be inserted into the tube and the position may beadjusted, and the transparent resin may be cured. In this case, a metaltube formed of stainless steel may be used as the tube.

In the above modification example, the example in which the light guidemember 155 is embedded into the tube 158 using the transparent resin 159and the light absorption member 157 is disposed at the distal end of thetransparent resin 159 has been described. However, the present inventionis not limited thereto. For example, a film having a light absorptionproperty may be used as the light absorption member 157 to cover thelight emitting portion 156, which is the light emitting surface of thelight guide member 155, with the film having a light absorptionproperty, and the light guide member 155 may be embedded into thetransparent resin. Alternatively, a gap may be provided between thelight emitting portion 156 of the light guide member 155 and the lightabsorption member 157, so that the light emitting portion 156 and thelight absorption member 157 face each other with the air layerinterposed therebetween.

In the modification example shown in FIG. 11, the example in which theinner needle 152 has the tube 158 has been described. However, thepresent invention is not limited thereto. For example, an inner needlemay be formed of a material having a light absorption property, forexample, black resin, and the light guide member 155 may be embeddedthereinside. In this case, the inner needle, in particular, the distalend portion of the inner needle also serves as the light absorptionmember 157 that absorbs light, which is emitted from the light emittingportion 156 of the light guide member 155, to generate an acoustic wave.Instead of embedding the light guide member 155 into the resin, thelight guide member 155 having almost the same outer diameter as theinner diameter of the insertion needle body 151 may be used, and thelight guide member 155 itself may be used as an inner needle. In thiscase, a film having a light absorption property, for example, a blackfluorine resin may be used as the light absorption member 157, so thatat least a part of the light guide member 155 including the lightemitting portion 156 is covered by the black fluororesin.

The light absorption member 157 is not essential. For example, the lightemitted from the light emitting surface of the light guide member 155may be emitted to the insertion needle body 151, and photoacoustic wavesmay be generated from a portion to which the light of the insertionneedle body 151 is emitted. In this case, the portion to which the lightof the insertion needle body 151 is emitted forms a photoacoustic wavegenerating portion. For example, photoacoustic waves may be generated inthe vicinity of the distal end of the insertion needle by emitting lightto the inner wall in the vicinity of the distal end of the insertionneedle body 151.

The insertion needle is not limited to being percutaneously insertedinto the subject from the outside of the subject, and a needle forultrasound endoscope may be used. The light guide member 155 and thelight absorption member 157 may be provided in the needle for ultrasoundendoscope, light may be emitted to the light absorption member 157provided in the distal end portion of the needle, and photoacousticwaves may be detected to generate a photoacoustic image. In this case,it is possible to insert the needle for ultrasound endoscope whilechecking the position of the distal end portion of the needle forultrasound endoscope by observing the photoacoustic image. Thephotoacoustic wave generated in the distal end portion of the needle forultrasound endoscope may be detected using a probe for body surface, ormay be detected using a probe built into the endoscope.

In the embodiment described above, the insertion needle 15 has beenconsidered as an insert. However, the insertion needle 15 is not limitedthereto. The insert may be a needle for radiofrequency ablation in whichan electrode used in radiofrequency ablation is housed, or may be acatheter inserted into the blood vessel, or may be a guidewire of thecatheter inserted into the blood vessel. Alternatively, the insert maybe an optical fiber for laser treatment.

Although the needle having an opening at the distal end is assumed as aneedle in each embodiment described above, the opening does notnecessarily need to be provided in the distal end portion. The needle isnot limited to a needle, such as an injection needle, and may be abiopsy needle used for a biopsy. That is, a biopsy needle that can beinserted into the inspection target of the body in order to sample thetissue in the inspection target may be used. In this case, photoacousticwaves may be generated in a sampling portion (inlet port) for samplingthe tissue of a biopsy part by sucking the tissue.

In FIG. 1, only one insertion needle 15 is drawn. However, the number ofinserts to be imaged in a photoacoustic image is not limited to one. Aplurality of sets of inserts and laser units corresponding thereto maybe prepared, and a photoacoustic image may be generated for each insertso that the position of each insert can be checked through thephotoacoustic image. During image display, the color of a photoacousticimage may be changed for each insert, and the photoacoustic image withthe changed color may be superimposed on the ultrasound image. In thiscase, it is possible to distinguish between a plurality of inserts inthe image.

Finally, FIG. 12 shows the appearance of a photoacoustic imagegeneration apparatus. The probe 11 is connected to the ultrasound unit12. The ultrasound unit 12 is configured as an integrated deviceincluding the image display means 14. The ultrasound unit 12 typicallyhas a processor, a memory, a bus, and the like. A program regardingphotoacoustic image generation is installed in the ultrasound unit 12.

The ultrasound unit 12 has a USB port 40. A USB connector including apower input terminal 41 and a trigger input terminal 42 of a laser unit13 is inserted into the USB port 40. In a case where the laser unit 13is a card-sized small and lightweight device, it is possible to hold theUSB connector by inserting the USB connector into the USB port of theultrasound unit 12. The USB port 40 may have any shape allowing a normalUSB connector to be inserted thereinto, and does not need to be a portfor transmitting and receiving a signal conforming to the normal USBstandard. In the USB port, a signal line for a trigger signal may beincluded instead of a digital signal line. That is, the USB port 40 maybe a USB type port as a connector having a total of four terminals oftwo lines for power supply and two lines for triggering. By using thesignal line for a trigger signal instead of the digital signal line, itbecomes easy to take trigger synchronization with the laser unit 13.

One end of the optical fiber that forms the light guide member 155(refer to FIG. 2) of the insertion needle 15 is connected to a lightoutput terminal 47 of the laser unit 13. The optical fiber is insertedinto the light output terminal 47, and is held by spring force or thelike. If the operator applies a strong force to the light outputterminal 47, for example, by pulling the insertion needle 15, theoptical fiber exits from the light output terminal 47. Accordingly, itis possible to prevent the optical fiber from being broken. In addition,by making it possible to directly insert or remove the optical fiberinto or from the light output terminal 47, there is an effect that thecost can be reduced without providing a connector in the optical fiberextending from the insertion needle 15.

Pulse energy of the pulsed laser light output from the laser unit 13 canbe set to 6.4 μJ if the core diameter of the optical fiber forming thelight guide member 155 is 200 μm. The pulse energy of the pulsed laserlight can be set to 2.0 μJ if the core diameter of the optical fiber is100 μm. The pulse time width can be set to 80 ns.

In FIG. 12, the light output terminal 47 is provided on a surfaceopposite to a surface on which the USB connector including the powerinput terminal 41 and the trigger input terminal 42 is present. However,it is preferable that the light output terminal 47 is provided on asurface perpendicular to the surface on which the USB connector ispresent. In a case where the USB connector and the light output terminal47 are provided on the opposite surfaces, if the laser unit 13 is pulledwhen the operator moves the insertion needle 15, the USB connector mayexit from the USB port 40. In contrast, in a case where the USBconnector and the light output terminal 47 are provided on the surfacesperpendicular to each other, the USB connector is difficult to exit fromthe USB port 40 even if the laser unit 13 is pulled.

In FIG. 12, the laser unit 13 is directly connected to the USB port 40.However, the present invention is not limited thereto, and the USB port40 and the laser unit 13 may be connected to each other using anextension cable or the like. The trigger input terminal 42 does not needto be included in the USB connector, and the laser unit 13 may acquire atrigger signal from a connector (terminal) different from the USB port40. For example, a trigger signal may be acquired from a connector forelectrocardiogram (ECG) measurement attached to the normal ultrasoundsystem. Alternatively, a trigger signal may be acquired from someterminals of the connector of the probe.

While the present invention has been described based on the preferredembodiment, the photoacoustic image generation apparatus is not limitedonly to the above embodiments, and various modifications and changes inthe configuration of the above embodiment are also included in the rangeof the present invention.

What is claimed is:
 1. A photoacoustic image generation apparatus, comprising: an insert at least a part of which is inserted into a subject and which has a light guide member for guiding light emitted from a light source, a light emitting portion for emitting light guided by the light guide member, and a photoacoustic wave generating portion for generating photoacoustic waves caused by light emitted from the light emitting portion; an acoustic wave detector including a probe for detecting photoacoustic waves emitted from the insert; and a controller configured to: generate a photoacoustic image based on a detection signal of the photoacoustic waves; extract a distal end candidate region from a shallow side based on a strength of the detection signal of the photoacoustic waves; and display the distal end candidate region that was extracted on image display.
 2. The photoacoustic image generation apparatus according to claim 1, wherein the controller extracts a distal end candidate region based on the strength of the detection signal.
 3. The photoacoustic image generation apparatus according to claim 1, the controller further configured to: when a plurality of the distal end candidate regions are extracted, suppressing a strength of a detection signal of photoacoustic waves in a distal end candidate region other than a distal end candidate region located in a shallowest portion in the photoacoustic image when imaging is realized, among the plurality of extracted distal end candidate regions, relative to a strength of a detection signal of photoacoustic waves in the distal end candidate region located in the shallowest portion.
 4. The photoacoustic image generation apparatus according to claim 3, wherein, with the distal end candidate region located in the shallowest portion among the extracted distal end candidate regions as a reference, the controller corrects a strength of a detection signal of photoacoustic waves in each distal end candidate region using a coefficient according to a distance in a depth direction between each distal end candidate region and the reference distal end candidate region.
 5. The photoacoustic image generation apparatus according to claim 4, wherein the controller emphasizes the distal end candidate region located in the shallowest portion relative to remaining distal end candidate regions.
 6. The photoacoustic image generation apparatus according to claim 4, wherein the distance is defined as a distance in the depth direction between a center-of-gravity position of the reference distal end candidate region and each of remaining distal end candidate regions.
 7. The photoacoustic image generation apparatus according to claim 4, wherein the coefficient is a maximum when the distance is 0, and a value of the coefficient decreases as the distance increases.
 8. The photoacoustic image generation apparatus according to claim 1, wherein the controller extracts, as a distal end candidate region, a region based on the strength of the detection signal of the photoacoustic waves.
 9. The photoacoustic image generation apparatus according to claim 1, wherein the controller extracts a distal end candidate region based not only on the strength of the detection signal of the photoacoustic waves but also on an area of a region that is determined based on the strength of the detection signal of the photoacoustic waves.
 10. The photoacoustic image generation apparatus according to claim 1, wherein the controller masks a region other than the extracted distal end candidate region in the photoacoustic image.
 11. The photoacoustic image generation apparatus according to claim 1, wherein the controller extracts the distal end candidate region after performing smoothing processing on the detection signal of the photoacoustic waves.
 12. The photoacoustic image generation apparatus according to claim 1, wherein the insert has an opening, and has an inner cavity thereinside.
 13. The photoacoustic image generation apparatus according to claim 12, wherein the light emitting portion emits light beams guided by the light guide member toward an inner wall of the inner cavity.
 14. The photoacoustic image generation apparatus according to claim 1, wherein the photoacoustic wave generating portion includes a light absorption member that absorbs the light emitted from the light emitting portion to generate photoacoustic waves.
 15. The photoacoustic image generation apparatus according to claim 1, wherein the insert is a needle inserted into a subject.
 16. The photoacoustic image generation apparatus according to claim 1, wherein the acoustic wave detector further detects reflected acoustic waves of acoustic waves transmitted toward a subject, and the controller is further configured to generate a reflected acoustic wave image based on the reflected acoustic waves is further provided.
 17. The photoacoustic image generation apparatus according to claim 16, wherein the controller displays the distal end candidate region that was extracted on the image display so as to be superimposed on the reflected acoustic wave image. 